The extensive cultivation such as soybean, corn, cotton and canola, among others, requires high investment in pesticides due to the quantity of plagues they harbour, especially arthropods, nematodes and molluscs. The damage caused by the different infestations individually may vary from one region to another, and the cost of their control can reach 35% of the production costs.
The utilization of chemical products has been the principal form of control of these infestations (Dias, D. G. S.; Soares, C. M. S. & Monnerat, R. G. Avaliação de larvicidas de Origem Microbiana no Controle de traça-das-cruciferas em Couve-Flor no Distrito Federal. Embrapa Recursos Genéticos e Biotecnologia, Comunicado Técnico no 74, 4 pgs, 2002), which requires huge investment, representing a burden or even making production impracticable in some cases (Castro, L. A. B. Plantas transgênicas resistentes a insetos: Perspectivas e limitações. Pesq. Agropec. Bras. Brasília, 27, S/N: 319-424, 1992). Moreover, these chemical products pollute the environment and can intoxicate the human beings.
In fact, it is evident the importance of simple and more effective forms of biological control in agriculture. The importance of pest control and the increasing public awareness of the direct and indirect effects of the pesticides in the environment and on public health have required new ways to control infestations and insects, which should to be more economical and less harmful to the ecosystem.
In the specific case of pest control, biological alternatives include the utilization of bacteria, fungi, viruses and even in the utilization of substances produced by the insect itself. These agents can be manipulated in order to increase their patogenicicity and to expand their range of action, industrial production or the incorporation of pesticide genes in plant species, leading to the production of transgenic plants (Perlak, F. J.; Deaton, R. W.; Armstrong, T. A.; Fuchs, R. L. Sims, S. R.; Greenplate, J. T. and Fischhoff, D. A. Insect resistent cotton plants. BioTechnology 8:939-963, 1990.).
From the many microbial agents that have pesticidal activity against insects and other invertebrates), we can highlight the bacteria belonging to the Bacillus, Brevibacillus and Paenibacillus genera. Species of these bacteria show a large geographical distribution and they are specific in controlling pests. One great advantage of their utilization is that they are harmless to man and domestic animals and they have no polluting effect to the environment The pesticide activity of the bacteria of the Bacillus, Brevibacillus and Paenibacillus genera is due to the production of toxins during the vegetative or sporulation phases of growth and by infection (Aronson, A. I., Beckman, W. y P. Dunn. Bacillus thuringiensis and related insect pathogens. Microbiol. Rev. 50, 1-24. 1986). The variability inside each species of these bacteria is very big. Recent records list more than 300 different toxins produced by B. thuringiensis. Due to it, laboratories all over the world are searching for strains with new toxins and other pathogenicity features that enable a higher availability of active principles which will be available for use in control strategies. Embrapa has a collection of 2.300 strains of entomopathogenic bacteria including bacteria from the genera Bacillus, Brevibacillus and Paenibacillus. Among the known strains, some have proven activity against arthropods, nematodes and molluscs.
Recently in the Laboratory of bacteriology of the Biological control Center of Embrapa Genetic Resources and Biotechnology studies were conducted to verify the possible utilization of insecticidal bacteria in a systemic way in plants.
The usage of biological agents to increase productivity and effectiveness in agriculture has been known for a long time. For example, the document U.S. Pat. No. 6,033,659 describes the use of a Bacillus cereus strain, named W35, and its mutants that are able to exert biological control through antibiotics and other toxins produced by this bacterium. According to this patent, plants, including seeds, cuttings and full-grown plants, treated with a sufficient quantity of this bacterium and its toxins or antibiotic produced by the bacterium itself, are protected against rotting or wilting of the root. The bacterium or its products are placed around the immediate neighborhood of the plant or the cutting or even by coating the seed with bacterial material.
The document U.S. Pat. No. 6,232,270 describes a composition comprising (a) an active ingredient effective in agriculture and (b) an enhancer containing a bacterial culture of Bacillus genus or a soil bacterium in the form of cells, cultures or suspensions, in sufficient quantity to improve the effectiveness of such active ingredient. Bacillus thuringiensis is mentioned in the list of enhancers. The enhancer component contains spores, cultures and suspensions of bacteria of the Bacillus genus or a soil bacterium. Preferentially, the enhancer component is in the spore form, as long as there is a appropriate adjustment of temperature, PH and salinity. Preferentially, the enhancer is a ATCC 55675 of the Bacillus cereus strain, which improves the effect of the growth plant regulator, Mepiquat chloride and of the Atrizine herbicide, that can be used to select the bacterial species, in order to improve other active ingredients such as systemic insecticides and fungicides. Bacteria found among roots of vigorously growing plants are also preferable as enhancer. The enhancer compound containing bacteria can be applied as a treatment or simultaneously with a variety of active ingredients such as herbicides, systemic pesticides and systemic fungicides. It is also mentioned that the foliar application of the local soil bacteria, at the rate of 0.1×1010 CFU/acre up to 10×1010 CFU/acre improves the natural mechanisms associated to the plant growth and propagation to a level sufficient for to the active ingredient to show the improved activity over or inside the treated plant.
It is obvious that the purpose of the use of the composition defined in the patent U.S. Pat. No. 6,232,270 is to improve plant growth and vigour and, in consequence, improve its resistance to pests. In that case, the bacterial component (spores, culture or suspensions of bacteria belonging to Bacillus genus) is just an improvement of the active component.
In addition, it is important to point out that, after foliar application of an agent to the treatment of plants, losses or the complete removal of the treatment agent can occur. For example, before absorption, bacteria may be washed from leaves as a result of rain and loss of activity can also occur as a result of ultraviolet rays (solar action), especially in the case of bioinseticides based on Bacillus thuringiensis (Bt) which requires high concentrations of endotoxins to be effective against the pests.
Thus, to increase productivity and effectiveness in pest control using biological agents, special attention must be paid to the form of application of the biopesticide to the plant and the quantity of bacteria applied to ensure the activity and permanence of the bacterium in association with the plant.
Thus, the studies mentioned above show that is not easy to obtain satisfactory results of plant treatment with Bacillus thuringiensis to control infestations of insects and other invertebrates through the inoculation and the susceptibility of the insects fed by these leaves as a result of the action of the bacterium. This success requires a deep knowledge about the interaction between the microorganism and the plant, about the strain utilized and about the pests' susceptibility to the Bacillus. This way, the knowledge of the Bt strains diversity, including biological and physicochemical characteristics of the produced endotoxins, is important to obtain appropriate formulations that ensure exposure of the target-pests to sufficient concentrations of lethal toxins for a long period.